Sputtering describes a number of physical techniques commonly used in, for example, the semiconductor industry for the deposition of thin films of various metals such as aluminum, aluminum alloys, refractory metal suicides, gold, copper, titanium, titanium-tungsten, tungsten, molybdenum, tantalum and less commonly silicon dioxide and silicon on an item (a substrate), for example a substrate or glass plate being processed. In general, the techniques involve producing a gas plasma of ionized inert gas "particles" (atoms or molecules) by using an electrical field in an evacuated chamber. The ionized particles are then directed toward a "target" and collide with it. As a result of the collisions, free atoms or neutral or ionized groups of atoms of the target material are released from the surface of the target, essentially liberating target material are released from the surface of the target, essentially liberating atomic-level particles from the target material. Many of the free particles which escape the target surface condense and form (deposit) a thin film on the surface of the object (e.g. wafer, substrate) being processed, which is located a relatively short distance from the target.
One common sputtering technique is magnetron sputtering. When processing substrates using magnetron sputtering, sputtering action is concentrated in the region of the magnetic field on the target surface so that sputtering occurs at a higher rate and at a lower process pressure than possible without the use of magnets. The target itself is electrically biased with respect to the substrate and chamber, and functions as a cathode. Objectives in engineering the cathode and its associated magnetic field source include uniform erosion of the target and uniform deposition of pure target material on the substrate being processed.
If, during sputtering, magnets generating a magnetic field are stationary at a location, then continuous sputtering consumes a disproportionate fraction of the sputtering target thickness at that location quickly and generates hot spots at the locations of sputtering. Therefore magnets are continuously moved across the back side of the target in a path designed to cause uniform utilization of the target's surface and sputter deposit a correspondingly uniform film thickness on the substrate being processed. Sputtering a target creates a deposition pattern on the substrate which generally matches the utilization (erosion) pattern on the target surface.
To avoid contamination of the processing chamber and substrate processed therein, sputtering is stopped before the non-uniform sputtering wear pattern has consumed the full thickness of the target material at any point. If any point on the plate behind the target were to be reached, sputtering of the target backing plate material (often copper) would occur, contaminating the vacuum chamber and the substrate being processed with the target backing material. Because of the non-uniform pattern of target utilization, sputtering is usually stopped when a large percentage of the target remains.
As the target erodes, the distance between the target surface (which is eroding away) and the substrate being sputtered is slowly increasing. The change in the distance between the target surface and the substrate being sputtered creates a change in the qualities of the sputtered material deposited and its uniformity. When material is deposited on large areas such as glass plates, variations in the thickness of deposited sputtered material are measurable and, may be unacceptable.
In generating the gas plasma and creating ion streams impacting on the cathode, considerable energy is supplied. This energy must be dissipated to avoid melting or nearly melting the structures and components involved. A common technique used for cooling sputtering targets is to pass water or other cooling liquid through a fixed internal passage of the sputtering target. Another cooling technique which is commonly used is to expose a back side of a target to a cooling bath. Cooling liquid circulating through the bath container assists in controlling the temperature of the back of the target assembly. A magnet assembly (magnetron) located on the back side of the target with a backside cooling bath moves within the liquid of the cooling bath.
FIGS. 1, 2, and 3 show a prior art sputtering chamber 50 in which a rectangular substrate 64 (shown in dashed lines in FIG. 1) is supported on a pedestal 52. A target assembly 58, consisting of a target backing plate 56 and a target 54 having a front face facing the pedestal 52, covers the upper flange of the processing chamber sealing it. On the side of the target assembly opposite from the pedestal 52 a magnetron chamber 60 encloses a magnetron assembly 62. The magnetron chamber 60 can be made vacuum tight to reduce the differential pressure across the target assembly 58 (with cooling fluid being routed through the target assembly), or it can be filled with cooling liquid to provide a cooling bath in contact with the back side of the target assembly 58. To enhance sputtering of a rectangular shaped substrate 64 (generally matching the shape of the outside of the chamber 50) the magnetron assembly 62 is a linear bar with rounded ends. The magnetron assembly 62 moves in a horizontal, back and forth (reciprocal) pattern within the magnetron chamber 62 as shown in by the arrows 68. The magnetron assembly passes through the magnetron chamber 62 and to the dashed outline of the magnetron assembly 62a. The outline of the area covered by magnetron movement is shown by the dashed line 66.
The magnetron assembly 62 as shown in FIG. 3 runs parallel to the target assembly 58 along one of a range of elevations between the low and high extremes (e.g., 96, 98), which are greatly exaggerated in this figure. The particular elevation (e.g., 96, 98) is dependent on the desired distance 92 from the front face of the target 54, which in turn determines the degree of sputtering enhancement desired for a particular process chamber pressure and sputtering process being used.
A conceptualized illustration of the magnetic field present around the strong Neodymium Boron Iron magnets used in the magnetron assembly is shown in the cross section of FIG. 4. The positive poles 72, 74, 76, 78 of the magnets shown, e.g., 70, are on the top (away from the sputtering target) in the outside loop 84 (FIG. 1) of permanent magnets and on the bottom (close to the sputtering target) in the inside loop 82 (FIG. 1), although the polarities may be reversed. A magnet backing plate 80 bridges the magnetic field on the top side of the magnetron thus preventing the magnetic field from extending up from the top side of magnetron assembly. In contrast, the magnetic field on the bottom side between adjacent magnets is conceptualized by the loops 86 showing a diminishing magnetic field strength farther down from the magnetron assembly 62. The loops of the magnetic field lines 86, portray a comparatively strong magnetic field in the loop 88 adjacent to the magnets, and drop off in the magnetic field strength rapidly as a function of the distance to a comparatively weakened magnetic field strength at the loop 90 farthest from the magnets. (The loops show an approximation of the diminution of the magnet field strength with distance). Any vertical movement of the magnetron assembly 62 that increases the distance between the front face of the target and the magnetron assembly 62 from the distance 92 (FIG. 3) to the distance 94 (FIG. 5), reduces the magnetic field strength at the surface of the target facing the pedestal 52 by a factor of approximately 5, relative to the range of field strength loops shown in FIG. 4.
FIG. 6 shows a target erosion profile for a target of 6061 Al in 2000 kilowatt hour power range. The contours shown by the plot show a generally uniform utilization of the target with a slight increase in erosion near the ends of the profile (a dwell location). The pattern observable from at the dwell locations corresponds to the shape of the magnet field emanating from magnetron assembly. The target erosion profile as shown here is related to the rate of deposition and film thickness uniformity or thickness control on a substrate being sputtered located opposite such a target (areas showing greater erosion on the target result in areas having greater deposition on the substrate). In this particular instance, there are two areas of relatively high erosion, one at the upper right corner 242 and the other at lower left hand corner 244 of FIG. 6, which produce corresponding deposition thickness anomalies on the substrate being sputtered.
The current specifications for target film thickness uniformity (even for large plates, such as the 50 by 60 centimeter plate shown in FIG. 6) is 5% or better. The anomalies of high erosion at the corners of the target as shown by the regions 242, 244 cause great concern in meeting the specification as they distort the film thickness uniformity so that a film thickness uniformity of only approximately 7% can be achieved. To improve uniformity the excessive erosion in the two regions 242, 244 must be reduced or eliminated so that the specification for film thickness uniformity can be met.
The observation of the high erosion in the corners has initiated a great deal of scrutiny without an identification of its true source. The positioning of an array of permanent magnets in the magnetron assembly assures a uniform magnetic field throughout the magnetron assembly. The general uniformity of the magnet field emanating from the magnetron is confirmed by the generally uniform erosion profile across the center of face of the target. Speculation about the source of the reason for the anomaly in the corners included research to determine whether a source of electrical or magnetic field anomalies could be identified. None has been identified.
FIG. 7 is a plot representing the film thickness on the surface of a substrate. It confirms the uniformity of the film thickness on the surface of a rectangular substrate. This plot shows an approximately mirror image correlation with the target erosion profile of FIG. 6.
In the field of thin film deposition, a size of substrates is becoming larger and larger since there is increasing need for larger size LCD screen. For example, current substrate size for production is up to 400 mm.times.500 mm, however, the size will be expanded up to 600 mm.times.700 mm or larger in the future.
One of the most difficult tasks in thin film deposition is how to achieve uniform deposition over a substrate. This shortcoming becomes the dominant factor preventing the economical production of larger and larger LCD screens.
The shortcomings in film thickness uniformity or thickness control of the existing sputtering target systems as described above continue to inhibit the wide use of sputtering as an efficient and cost-effective means for applying surface coatings on large substrates.